The invention relates to a device for displaying pictures by means of a cathode-ray tube which comprises in an evacuated envelope a display screen and an electron gun for generating an electron beam, which cathode-ray tube has a main system of deflection coils provided around the tube for deflecting the electron beam over the display screen in two mutually transverse directions.
Such a device is known from Netherlands Patent Application No. 7707008 laid open to public inspection on Dec. 12, 1978, in which a device is shown having a cathode-ray tube for projection television purposes.
Deflection coils used in cathode-ray tubes often cause electron optical aberrations in the deflected electron beam. The most important aberrations are astigmatism, curvature of the field and coma. These are the so-called third-order aberrations. In addition to these third-order aberrations, picture defects of the fifth and seventh order also occur. However, these are of importance only at large values of the distance s of the electron beam from the axis of the system of deflection coils in the deflection field and of the tangent of the deflection angle .phi.. These aberrations can be corrected by using dynamic corrections synchronously with the deflection. For example, astigmatism is corrected by means of one or two dynamic quadrupoles, curvature of the field is corrected by means of dynamic focusing and coma errors are corrected by means of dynamic sexapole, octapole and decapole fields.
From the article "A practical approach to the third-order theory of magnetic deflection and its application to the deflection of convergent electron beams", Digest of Technical Papers of the International Symposium 1980 of the Society for Information Display, San Diego, Calif., which may be considered to be incorporated herein by reference, it follows that, when the horizontal and vertical deflection fields are substantially identical two-pole fields having an effective length EQU l.sub.3 =L/2 (1)
wherein L is the distance between the deflection point of the system of deflection coils to the display screen, no third-order astigmatism occurs. Moreover it is stated that the effective length must be taken to be slightly larger than shown in (1) in connection with fifth-order aberrations. The effective length l.sub.3 is determined by the length and the shape of the magnetic field and is defined in P. E. Kaus, RCA Rev. 17, 168, 1956 ##EQU1## where H.sub.d is the generated magnetic field as a function of the place Z on the axis of the system of deflection coils. In short cathode-ray tubes, however, it is not possible to satisfy the requirement as indicated in (1) because the system of deflection coils must be provided comparatively close to the display screen. It is also attractive when the place of the deflection point can be chosen to be more or less independent of the coil dimensions and coil shape.